1. Field of the Invention
The present invention relates to an electrophotographic method used for the image-forming apparatuses such as copiers, laser printers and plain paper facsimiles. More specifically, the invention relates to an electrophotographic method capable of executing an image-forming processing at a high speed.
2. Description of the Prior Art
In an image-forming apparatus utilizing an electrophotographic method as represented by a copier, a rotary photosensitive material drum is subjected to an image-forming processing which includes main electric charging, exposure to image-bearing light, developing, transfer, cleaning and removal of electric charge, in order to form an image.
A variety types of drum-like photosensitive materials have been used for the image-forming process based on the electrophotographic method. In recent years, in particular, there has been widely used an organic photosensitive material having an organic photosensitive layer formed on an electrically conducting blank tube. That is, the organic photosensitive material has a great advantage in that an organic photosensitive layer can be easily formed by a coating method at a decreased cost. Many organic photosensitive materials can be used for forming the organic photosensitive layer. By using a less harmful compound, the user is allowed to dispose of the photosensitive material when it is out of service life, offering a so-called maintenance-free advantage.
The above organic photosensitive materials can be grouped into single-layer organic photosensitive materials in which a photosensitive layer is a single layer containing an electric charge-generating agent and an electric charge-transporting agent, and function-separated laminated organic photosensitive materials in which the photosensitive layer includes an electric charge-generating layer containing an electric charge-generating agent and an electric charge-transporting layer containing an electric charge-transporting agent.
In modern high-speed digital image-forming apparatuses employing the above image-forming process that works at high speeds in a digital manner, however, various problems are occurring being caused by a high-speed rotation of the photosensitive material drum and due to the use of a semiconductor laser as a source of light for exposure to image-bearing light.
For example, when the function-separated laminated organic photosensitive material is exposed to image-bearing light by using a semiconductor laser as a source of light, interference fringes (Moire fringes) easily occur being affected by the surface of the electrically conducting blank tube which is the photosensitive layer-forming surface. In order to prevent the occurrence of interference fringes, a method has been known to set the surface roughness of the electrically conducting blank tube to lie within a predetermined range leaving, however, a problem that results from the high-speed rotation of the photosensitive material drum. That is, in the image-forming process, a cleaning blade is brought into pressed contact with the surface of the photosensitive material drum in order to remove the toner remaining on the surface of the photosensitive material drum. With the cleaning blade being brought into pressed contact with the surface of the photosensitive material drum, however, the electric charge-transporting layer on the surface of the photosensitive material drum is worn out to deteriorate the image properties to a conspicuous degree.
In the single-layer organic photosensitive material, on the other hand, light in the step of exposure to light is almost all absorbed by the surface of the photosensitive layer or near the surface thereof, and the interference fringes develop very little and the wear of the photosensitive layer is not much of a problem. That is, in the single-layer organic photosensitive material, the thickness of the photosensitive layer is considerably larger than that of the electric charge-transporting layer in the function-separated laminated photosensitive material. Therefore, wear of the photosensitive layer does not much affect the image properties. Thus, the single-layer organic photosensitive material is suited for the digital image-forming process of a high speed. Even by using the above organic photosensitive material, however, limitation is imposed on the processing speed for favorably forming images for extended periods of time. For example, when the image-forming process is conducted by rotating the photosensitive material drum at a peripheral speed of not smaller than 400 mm/sec, the photosensitive material drum is quickly accelerated up to a predetermined peripheral speed at the start of image formation, and a strong static frictional force acts on the tip of the cleaning blade that comes in contact with the surface of the photosensitive layer. Even in a steady state where the photosensitive material drum is rotating at a constant peripheral speed, a strong centrifugal force acts due to a high rotational speed, and a large dynamic frictional force acts on the tip of the cleaning blade. As a result, the tip of the blade is burred (the blade edge is finely cut and is burred) and, in an extreme case, the tip of the blade is turned up, causing the cleaning to become defective and making it difficult to favorably form the images within short periods of time.
Thus, no means has yet been proposed for avoiding defective cleaning of when the image-forming process is conducted at a high speed.
It is therefore an object of the present invention to provide an electrophotographic method which is capable of effectively preventing the defective cleaning at the time when an image-forming process is conducted by using a single-layer organic photosensitive material drum and by rotating the photosensitive material drum at a peripheral speed of not smaller than 400 mm/sec.
Another object of the present invention is to provide an electrophotographic method capable of favorably forming images without interference fringes even when a digital image-forming process is executed at a high speed by effecting the exposure to image-bearing light by using a semiconductor laser as a source of light.
According to the present invention, there is provided an electrophotographic method for forming image by subjecting a rotary photosensitive material drum to an image-forming process which includes main electric charging, exposure to image-bearing light, developing, transfer, cleaning and removal of electric charge, wherein:
said image-forming process is conducted by rotating the photosensitive material drum at a speed of not lower than a peripheral speed of 400 mm/sec;
said photosensitive material drum is a single-layer organic photosensitive material drum having a single organic photosensitive layer of a thickness of not smaller than 25 xcexcm formed on an electrically conducting blank tube having a surface roughness Ra as defined in JIS-B-0601) of not larger than 0.6 xcexcm; and
the cleaning is effected by using a cleaning blade that is brought into pressed contact with the surface of the photosensitive material drum.
JIS-B-0601 describes a surface roughness Ra determined by the following Formula (I)
Ra=1/L∫0L|f(x)|dxxe2x80x83xe2x80x83Formula (I) 
In particular, an arithmetic average roughness (Ra)L is taken along a standard length (L) obtained at random from a surface to be measured. The standard length is then taken along a roughness curve well known to those of ordinary skill in the art and as shown in FIG. 3. The length is given in the direction of the removed portion and an Y-axis established in the direction of lengthwise magnification, wherein the roughness curve is expressed as y=f(x).
In the present invention, a distinguished feature resides in the image-forming process by using the single-layer organic photosensitive material drum having a single organic photosensitive layer of a thickness of not smaller than 25 xcexcm formed on the electrically conducting blank tube of a smooth surface with the surface roughness Ra of not larger than 0.6 xcexcm and by rotating the photosensitive material drum at a peripheral speed of not lower than 400 mm/sec. Use of the single-layer organic photosensitive material drum effectively suppresses the formation of burrs and turn-up of the cleaning blade even when the high-speed image-forming process is conducted by rotating the photosensitive material drum at a peripheral speed of not lower than 400 mm/sec, and makes it possible to favorably form images for extended periods of time without permitting the cleaning to become defective.
Table 1 below shows the condition of the cleaning blade of when the image-forming process is executed for consecutively obtaining 100,000 pieces of copies by using a single-layer organic photosensitive material drum having a single photosensitive layer of a thickness of 20 xcexcm formed on the surface of an electrically conducting blank tube (made of aluminum) having a surface roughness Ra of 1.3 xcexcm while varying the rotational speed (peripheral speed) of the drum. The single-layer organic photosensitive material drum used here is employed by the conventional image-forming apparatus, and exposure to image-bearing light is conducted by using a semiconductor laser as a source of light.
As will be understood from Table 1 above, when there is used a single-layer organic photosensitive material drum having a surface roughness of the electrically conductive blank tube and having a thickness of the photosensitive layer lying outside the ranges of the present invention, damage to the cleaning blade becomes conspicuous as the rotational speed of the photosensitive material drum increases. In particular, damage to the blade becomes more conspicuous as the peripheral speed of the photosensitive material drum exceeds 400 mm/sec, and the image becomes defective within short periods of time due to defective cleaning.
On the other hand, when there is used a single-layer organic photosensitive material drum having a surface roughness Ra of the electrically conducting blank tube of not larger than 0.6 xcexcm and a thickness of the photosensitive layer of not smaller than 25 xcexcm, the image-forming process can be stably conducted for extended periods of time by rotating the photosensitive material drum at a peripheral speed of not lower than 400 mm/sec without damaging the cleaning blade and without causing the image to become defective that results from the defective cleaning.
It has been taught in, for example, FIG. 3 of Japanese Unexamined Patent Publication (Kokai) No. 305044/1996 that the surface roughness of the electrically conducting blank tube is reflected on the surface of the organic photosensitive layer formed on the surface of the blank tube, and the surface of the organic photosensitive layer can be made smooth by decreasing the surface roughness of the blank tube. Upon setting the surface roughness of the electrically conducting blank tube to be smaller than a predetermined value, therefore, it can be expected to prevent damage to the cleaning blade caused by friction by the photosensitive layer. Unexpectedly, however, it was learned that simply setting the surface roughness of the electrically conducting blank tube to be smaller than a predetermined value to make the surface of the photosensitive layer smooth, is not enough for preventing damage to the cleaning blade when the image is formed at high speeds. That is, the surface state of the electrically conducting blank tube is more reflected on the surface of the photosensitive layer when the thickness of the photosensitive layer is small. Therefore, if damage to the cleaning blade could be prevented by making the surface of the photosensitive layer smooth, then, it can be expected that damage to the blade can be prevented by setting the surface roughness of the electrically conducting tube to be smaller than a predetermined value and decreasing the thickness of the photosensitive layer. When the surface roughness Ra of the electrically conductive blank tube is set to be 0.25 xcexcm like in Example 1 and the thickness of the photosensitive layer to be 20 xcexcm which is smaller than that of Example 1, however, the cleaning blade is damaged within short periods of time at a drum peripheral speed of 400 mm/sec as demonstrated by Comparative Example 5 appearing later. That is, the blade edge is burred after the image-forming cycle of 6000 copies, and the image becomes defective after 7500 copies due to defective cleaning.
According to the present invention in which the surface roughness Ra of the electrically conductive blank tube is set to be smaller than the above-mentioned predetermined value and the thickness of the photosensitive layer is set to be not smaller than 25 xcexcm, however, the cleaning blade is not damaged in the high-speed image-forming cycles, and defective cleaning and defective image are effectively avoided (in Example 1 appearing later, for example, the cleaning blade is not damaged and defective cleaning does not occur even when the image-forming cycle is conducted until 100,000 copies are obtained at a drum peripheral speed of 1000 mm/sec.). It is quite an unexpected event that the cleaning blade is not damaged by friction upon setting the thickness of the photosensitive layer to a large value so will not to reflect the surface roughness of the blank tube while setting the surface roughness Ra of the electrically conducting blank tube to be smaller than a predetermined value. It is presumed that in a high-speed image-forming cycle with a drum peripheral speed of 400 mm/sec, the frictional force exerted on the cleaning blade is so large that making the surface of the photosensitive layer smooth is not enough for preventing damage to the blade, but damage to the blade is effectively prevented by increasing the thickness of the photosensitive layer to some extent so that the photosensitive layer exhibits a cushioning function.
Simply making the surface of the photosensitive layer smooth is not enough for preventing damage to the blade as will be obvious from the experimental results of Comparative Example 6 appearing later. That is, Comparative Example 6 deals with the execution of image-forming cycles by using an organic photosensitive material drum (drum of Comparative Example 1) having a photosensitive layer of a thickness of 25 xcexcm formed on an electrically conducting blank tube with a surface roughness Ra of 0.80 xcexcm and by adjusting the surface roughness Ra of the photosensitive layer to be not larger than 0.5 xcexcm by using a polishing agent. If damage to the blade could be prevented by making the surface of the photosensitive layer smooth, then, damage to the blade could be prevented in Comparative Example 6, too. In Comparative Example 6, however, the cleaning blade is damaged after the image-forming cycles of 20,000 copies at the drum peripheral speed of 400 mm/sec, and the image becomes defective after 35,000 copies due to defective cleaning. From the above, it is believed that simply making the surface of the photosensitive layer smooth is not capable of effectively preventing damage to the blade and that setting the surface roughness Ra of the electrically conducting blank tube to be not larger than 0.6 xcexcm exhibits action in addition to making the surface of the photosensitive layer smooth (the present inventors presume that adhesion is improved between the photosensitive layer and the surface of the blank tube).
According to the present invention as described above, the photosensitive layer is formed on the electrically conductive blank tube having a surface roughness Ra of not larger than 0.6 xcexcm to make the surface of the photosensitive layer flat to a suitable degree and, at the same time, to enhance adhesion between the photosensitive layer and the electrically conducting blank tube. Besides, the thickness of the photosensitive layer is selected to be not smaller than 25 xcexcm so that the photosensitive layer works as a cushioning layer. This effectively prevents damage to the cleaning blade caused by friction in a high-speed image-forming cycle at a drum peripheral speed of 400 mm/sec, effectively suppresses the occurrence of defective cleaning and defective image caused by the damaged blade and, hence, makes it possible to stably form images over extended periods of time.
Here, in the present invention, when a function-separated laminated photosensitive layer is formed on the electrically conducting blank tube instead of forming the single-layer organic photosensitive layer, there develop interference fringes when the photosensitive layer is exposed to image-bearing light particularly when a semiconductor laser is used as a source of light.